This invention relates to the electrodeposition of copper from aqueous acidic baths. More particularly this invention relates to a high-throw aqueous acidic bath for the electrodeposition of copper containing additives to produce printed circuit boards having plated through-holes.
A large number of agents have been described in the art for use in electroplating baths alone or in combination to improve the quality of the electrodeposit in terms of brightness, surface smoothness, hardening, leveling and to increase the lower limiting current density of deposition. The use of such agents in aqueous, acidic, copper plating baths for the preparation of printed circuits is described in Chapter 7 of the "Printed Circuits Handbook", Second Edition, 1979, McGraw-Hill Book Company, edited by Clyde F. Coombs, Jr., and in particular Sections 18 and 19. In Section 18, Coombs indicates that additives to acid copper sulfate plating baths can be effective in grain refinement, leveling, and hardening and as a brightener or a means of increasing the current density range. The term "leveled" denotes a copper deposit whose surface is smoother than its substrate. The term "bright" indicates that the formed electrodeposit is characterized by having a highly reflective surface gloss over most of its surface. Generally leveling and brightness vary with the current density at the cathode, all other factors such as copper salt concentration, pH, type of acid, temperature etc., being equal. As the current density decreases, brightness of the electrodeposit tends to decrease often diminishing to a haze. The strength of leveling also varies with current density. Coombs indicates that such additives include glue, peptone, recorcinol, thiourea, molasses, gum arabic as well as proprietary compositions.
A variety of brightening and leveling additives are disclosed in U.S. Pat. Nos. 3,502,551; 4,376,685 and 4,555,315 and the patents cited therein.
As a result of current trends in printed circuit fabrication, i.e., use of surface mount technology, multilayer printed circuit boards are becoming thicker, circuit density is becoming greater and hole aspect ratios are likewise becoming larger. Hole aspect ratio is the ratio of the thickness of a printed circuit board to the diameter of the through hole therein. In a publication entitled "Electroplating High Aspect Ratio Through Holes in Thick Printed Circuit Boards" (Metal Finishing, vol 68, pages 50-54, 1990), Fisher et al. reported on difficulties associated with the plating process and efforts to optimize the acid copper bath and they make recommendations for printed through hole plating processes, equipment, and controls to enable successful plating of high aspect ratio through holes, e.g., boards 5.8 mm thick with an aspect ratio of 18:1. In their discussion of the electrochemistry for plating circuit boards with through holes, "throwing power" is defined as the ratio of the hole center copper deposit thickness to its thickness on the surface of the board and indicate that this ratio should be greater than 0.67:1 for plating high aspect ratio holes in printed circuit boards. Plating baths involved in such electrochemistry, are generally known in the industry as "high-throw" plating baths. Typical high-throw copper plating baths are highly acidic, e.g., a pH of about 0, and contain substantially less copper ion concentrations compared to normal acid, copper electroplating baths. To achieve high throwing power, current densities of less than 1.1 A/dm.sup.2 typically are required.
The high acid concentrations used in commercially available high throw processes make it difficult to consistently produce bright deposits. Existing processes also require plating to occur at very low current densities in order to get acceptable throwing power.
There is a need which is not met by present high-throw technology to increase the consistency of performance and throughput of printed circuit boards by plating at higher current densities without sacrificing throwing power.